The minimum time in which a full carafe of coffee (one-half gallon) can be made in an automatic drip coffee maker is two and one-half minutes. Any faster flow rate through the coffee grounds results in incomplete extraction. In order to make coffee in this minimum time, it is necessary that the water be preheated and held at correct brewing temperature (approximately 200.degree. F.) until such time as the brewing cycle is to be commenced.
All present day coffee makers, which utilize preheated water, operate on the same principle: That of cold water displacement of preheated water. More specifically, a large quantity of water is preheated by immersion heaters and thermostatically maintained at correct brewing temperature by an immersion thermostat. To make coffee, a quantity of cold water is introduced, via a holding pan and inlet tube, to the bottom of the preheated water whereupon an equal quantity of the preheated water is displaced from the top thereof, via an outlet tube, to flow to an underlying filter basket.
The quantity of preheated water is necessarily large as compared with the quantity of cold water to be added to the bottom of the preheated water to insure that the displaced, preheated water is not cooled by the cold water addition. Typically, the ratio of preheated water to cold water addition is 3:1. The capacity of the preheat water tank would thus be one and one-half gallons, while that of the holding pan is one-half gallon. The total required water capacity that must be accomodated within the appliance housing is, then, two gallons.
This type coffee maker, when new, has two distinct advantages over other automatic drip coffee makers; speed and brew quality. An unusually large filter basket is typically employed and the hot water is introduced at such a rate that the coffee grounds are literally floated in a large quantity of hot water. The inpour rate to the filter basket and the outflow rate therefrom (as metered by the size of the inflow and outflow openings) is such as to assure that the hot water/coffee suspension is just short of overflowing the filter basket until all the hot water has been introduced. The key to the superior coffee quality is that the initial brewing contact is between a large body of water at correct brewing temperature and all of the coffee grounds. A flow-through time of approximately two and one-half minutes is obtainable with this type coffee maker.
The purposes of the present invention are to achieve the brew quality and brew time advantages mentioned above in a much smaller appliance profile requiring only that one-half gallon of water be preheated; to eliminate the use of small bore tubing and its attendant liming problems and to remove from water contact the necessary heating element and thermostatic control.